Remove unused variable.
Add Target API to determine if an include is a system include.

The implementation can be modified later so that system includes
can be determined on a per-target basis.
Store system include directories in the cmTarget.

Entries from the cmMakefile are processed and maintained similarly
to other include directories. The include_directories(SYSTEM)
signature affects all following targets, and all prior targets
in the same makefile.
Extend the cmTargetPropCommandBase interface property handling.

This can be used to handle INTERFACE SYSTEM include directories
in particular.
Add a SYSTEM parameter to target_include_directories (#14180)

This is similar to the include_directories(SYSTEM) signature
in that it allows telling the compiler to ignore warnings from
such headers.
Add the INTERFACE_SYSTEM_INCLUDE_DIRECTORIES target property.

Unlike other target properties, this does not have a corresponding
non-INTERFACE variant.

This allows propagation of system attribute on include directories
from link dependents.
Merge topic 'tid-system-argument' into next

2d6f9bf Add the INTERFACE_SYSTEM_INCLUDE_DIRECTORIES target property.
1925cff Add a SYSTEM parameter to target_include_directories (#14180)
286f227 Extend the cmTargetPropCommandBase interface property handling.
83498d4 Store system include directories in the cmTarget.
f1fcbe3 Add Target API to determine if an include is a system include.
2679a34 Remove unused variable.